1. Field of the Invention
The invention relates to a high-pressure fuel supply apparatus for an internal combustion engine.
2. Description of the Related Art
For example, in a fuel system for an in-cylinder injection internal combustion engine, a high-pressure fuel pump, which increases a pressure of fuel, is employed to inject the fuel directly into a cylinder. In the fuel system, the fuel supplied from a fuel tank by a low-pressure fuel pump is pressurized by the high-pressure fuel pump so that the fuel is delivered under pressure to a fuel injection valve through a high-pressure fuel passage and a delivery pipe. A fuel pressure in the delivery pipe is adjusted, and maintained at a predetermined high pressure by, for example, a high-pressure regulator.
In the configuration in which the pressure in the delivery pipe is maintained at a high pressure, after the engine is stopped, the pressure in the delivery pipe may not be decreased, that is, the pressure in the delivery pipe may be maintained at a high pressure. Therefore, the fuel may leak from the fuel injection valve. The fuel that leaks from the fuel injection valve is accumulated in the cylinder, and the accumulated fuel, which remains unburned, is discharged when the engine is started next time. This may result in deterioration of emissions at an engine start time. For example, Japanese Patent Application Publication No. 2006-90222 describes a configuration in which a discharge valve with a leak function is provided between the high-pressure fuel pump and the delivery pipe to suppress the leakage of the fuel. The discharge valve with the leak function is provided in the high-pressure fuel passage. The discharge valve allows the fuel to pass through the discharge valve only in the direction from the high-pressure fuel pump toward the delivery pipe. Pores, which are constantly open, are provided in the discharge valve. In the configuration, when the engine is stopped while the pressure at the side where the delivery pipe is provided is higher than the pressure at the side where the high-pressure fuel pump is provided, the high-pressure fuel at the side where the delivery pipe is provided is returned toward the high-pressure fuel pump through the pores. As a result, the fuel pressure in the delivery pipe is decreased. Thus, it is possible to suppress the leakage of the fuel.
The fuel is returned toward the high-pressure fuel pump through the discharge valve with the leak function described in the publication No. 2006-90222, until the pressure at the side where the delivery pipe is provided is equal to the pressure at the side where the high-pressure fuel pump is provided. Therefore, it is difficult to appropriately adjust the pressure at the side where the delivery pipe is provided.
If the pressure in the delivery pipe decreases after the engine is stopped, a temperature at which the fuel is vaporized (i.e., a vaporization temperature) also decreases. Further, the temperature of the fuel in the delivery pipe increases because a temperature in an engine room increases, for example, due to stop of an engine cooling system. As a result, the temperature of the fuel in the delivery pipe may exceed the vaporization temperature at which the fuel is vaporized. Thus, vapor (bubbles) may be generated in the delivery pipe. That is, in the configuration described in the publication No. 2006-90222, although the leakage of the fuel from the fuel injection valve is suppressed, the vapor may be generated in the delivery pipe. If the vapor is generated in the delivery pipe, the vapor is retained in the delivery pipe, or discharged from the fuel injection valve together with the fuel. This may result in deterioration of startability of the internal combustion engine.